arXiv subjects

A. Valli

Publications and source records attributed to A. Valli.

15 recordsLinked to original sources

arXiv2023

Stability of destructive interference antiresonances in electron transport through graphene nanostructures

We investigate the stability of destructive quantum interference (DQI) in electron transport through graphene nanostructures connected to source and drain electrodes. The fingerprint of DQI is the presence of an antiresonance in the transmission function, and its origin is deeply connected to the topology of the atomic structure, which we discuss in terms of symmetry arguments supported by numerical simulations. A systematic analysis of the influence of system size on the transmission function reveals that the DQI antiresonance persists for large systems in the ballistic regime and establishes the quantum confinement gap as the intrinsic resolution limit to detect QI effects. Furthermore, we consider the influence of disorder, electron-electron and electron-phonon interactions, and provide quantitative criteria for the robustness of DQI in their presence. We find that the conductance is quite sensitive to perturbations, and its value alone may not be sufficient to characterize the QI properties of a junction. Instead, the characteristic behavior of the transmission function is more resilient, and we suggest it retains information on the presence of an antiresonance even if DQI is partially concealed or suppressed. At the same time, DQI results in a non-linear transport regime in the current-bias characteristics that can be possibly detected in transport experiments.

cond-mat.mes-hall↗

arXiv2022

Strongly correlated physics in organic open-shell quantum systems

Strongly correlated physics arises from electron-electron scattering within partially filled orbitals. Organic molecules in open-shell configurations are therefore good candidates to exhibit many-body effects. We focus on electron transport in a two-terminal single-molecule junction setup, in which the molecular bridge consists of an organic radical with a molecular orbital hosting a single unpaired electron (SOMO). We perform beyond state-of-the-art numerical simulations combining an ab-initio description of the chemical environment, with quantum field-theoretical techniques that account for many-body effects. The key observation is that the SOMO resonance is prone to splitting and we identify a giant electronic scattering rate as the driving many-body mechanism, akin to that of the Mott metal-to-insulator transition. By comparing linear and cyclic radicals, we show that the spatial distribution of the SOMO and its projection on the molecular backbone have dramatic consequences for the transport properties of the junction. We argue that the phenomenon and the underlying microscopic mechanism apply to a broad family of open-shell molecular systems, and can explain puzzling experimental observations such as suppressed conductance in radical junctions.

cond-mat.str-el↗

arXiv2022

Electrode effects on the observability of destructive quantum interference in single-molecule junctions

Destructive quantum interference (QI) has been a source of interest as a new paradigm for molecular electronics as the electronic conductance is widely dependent on the occurrence or absence of destructive QI effects. In order to interpret experimentally observed transmission features, it is necessary to understand the effects of all components of the junction on electron transport. We perform non-equilibrium Green's function calculations within the framework of density functional theory to assess the structure-function relationship of transport through pyrene molecular junctions with distinct QI properties. The chemical nature of the anchor groups and the electrodes controls the Fermi level alignment, which determines the observability of destructive QI. A thorough analysis allows to disentangle the transmission features arising from the molecule and the electrodes. Interestingly, graphene electrodes introduce features in the low-bias regime, which can either mask or be misinterpreted as QI effects, while instead originating from the topological properties of the edges. Thus, this first principles analysis provides clear indications to guide the interpretation of experimental studies, which cannot be obtained from simple Hückel model calculations.

cond-mat.mes-hall↗

arXiv2022

Enhancing the sensitivity and selectivity of pyrene-based sensors for detection of small gaseous molecules via destructive quantum interference

Graphene-based sensors are exceptionally sensitive with high carrier mobility and low intrinsic noise, and have been intensively investigated in the past decade. The detection of individual gas molecules has been reported, albeit the underlying sensing mechanism is not yet well understood. We focus on the adsorption of NO$_2$, H$_2$O, and NH$_3$ on a molecular junction with a pyrene core, which can be considered as a minimal graphene-like unit. We systematically investigate the chemiresistive response within the framework of density functional theory and non-equilibrium Green's functions. We highlight the fundamental role of quantum interference (QI) in the sensing process, and we propose it as a paradigmatic mechanism for sensing. Owing to the open-shell character of NO$_2$, its interaction with pyrene gives rise to a Fano resonance thereby triggering the strongest chemiresistive response, while the weaker interactions with H$_2$O and NH$_3$ result in lower sensitivity. We demonstrate that by exploiting destructive QI arising in the meta-substituted pyrene, it is possible to calibrate the sensor to enhance both its sensitivity and chemical selectivity by almost two orders of magnitude so that individual molecules can be detected and distinguished. These results provide a fundamental strategy to design high-performance chemical sensors with graphene functional blocks.

cond-mat.mes-hall↗

arXiv2020

Smart local orbitals for efficient calculations within density functional theory and beyond

Localized basis sets in the projector augmented wave formalism allow for computationally efficient calculations within density functional theory (DFT). However, achieving high numerical accuracy requires an extensive basis set, which also poses a fundamental problem for the interpretation of the results. We present a way to obtain a reduced basis set of atomic orbitals through the subdiagonalization of each atomic block of the Hamiltonian. The resulting local orbitals (LOs) inherit the information of the local crystal field. In the LO basis, it becomes apparent that the Hamiltonian is nearly block-diagonal, and we demonstrate that it is possible to keep only a subset of relevant LOs which provide an accurate description of the physics around the Fermi level. This reduces to some extent the redundancy of the original basis set, and at the same time it allows one to perform post-processing of DFT calculations, ranging from the interpretation of electron transport to extracting effective tight-binding Hamiltonians, very efficiently and without sacrificing the accuracy of the results.

cond-mat.mtrl-sci↗

arXiv2018

Emergent $\text{D}_6$ symmetry in fully-relaxed magic-angle twisted bilayer graphene

We present a tight-binding calculation of a twisted bilayer graphene at magic angle $θ\sim 1.08^\circ$, allowing for full, in- and out-of-plane, relaxation of the atomic positions. The resulting band structure displays as usual four narrow mini bands around the neutrality point, well separated from all other bands after the lattice relaxation. A thorough analysis of the mini-bands Bloch functions reveals an emergent $D_6$ symmetry, despite the lack of any manifest point group symmetry in the relaxed lattice. The Bloch functions at the $Γ$ point are degenerate in pairs, reflecting the so-called valley degeneracy. Moreover, each of them is invariant under C$_{3z}$, i.e., transforming like one-dimensional, in-plane symmetric irreducible representation of an "emergent" $D_6$ group. Out of plane, the lower doublet is even under C$_{2x}$, while the upper doublet is odd, which implies that at least eight Wannier orbitals, two $s$-like and two $p_z$-like for each of the two supercell sublattices AB and BA are necessary, probably not sufficient, to describe the four mini bands. This unexpected one-electron complexity is likely to play an important role in the still unexplained metal-insulator-superconductor phenomenology of this system.

cond-mat.str-el↗

arXiv2018

Coexistence of metallic edge states and anti-ferromagnetic ordering in correlated topological insulators

We investigate the emergence of anti-ferromagnetic ordering and its effect on the helical edge states in a quantum spin Hall insulator, in the presence of strong Coulomb interaction. Using dynamical mean-field theory, we show that the breakdown of lattice translational symmetry favours the formation of magnetic ordering with non-trivial spatial modulation. The onset of a non-uniform magnetization enables the coexistence of spin-ordered and topologically non-trivial states. An unambiguous signature of the persistence of the topological bulk property is the survival of bona fide edge states. We show that the penetration of the magnetic order is accompanied by the progressive reconstruction of gapless states in sub-peripherals layers, redefining the actual topological boundary within the system.

cond-mat.str-el↗

arXiv2017

Realistic theory of electronic correlations in nanoscopic systems

Nanostructures with open shell transition metal or molecular constituents host often strong electronic correlations and are highly sensitive to atomistic material details. This tutorial review discusses method developments and applications of theoretical approaches for the realistic description of the electronic and magnetic properties of nanostructures with correlated electrons. First, the implementation of a flexible interface between density functional theory and a variant of dynamical mean field theory (DMFT) highly suitable for the simulation of complex correlated structures is explained and illustrated. On the DMFT side, this interface is largely based on recent developments of quantum Monte Carlo and exact diagonalization techniques allowing for efficient descriptions of general four fermion Coulomb interactions, reduced symmetries and spin-orbit coupling, which are explained here. With the examples of the Cr (001) surfaces, magnetic adatoms, and molecular systems it is shown how the interplay of Hubbard U and Hund's J determines charge and spin fluctuations and how these interactions drive different sorts of correlation effects in nanosystems. Non-local interactions and correlations present a particular challenge for the theory of low dimensional systems. We present our method developments addressing these two challenges, i.e., advancements of the dynamical vertex approximation and a combination of the constrained random phase approximation with continuum medium theories. We demonstrate how non-local interaction and correlation phenomena are controlled not only by dimensionality but also by coupling to the environment which is typically important for determining the physics of nanosystems.

cond-mat.str-el↗

arXiv2016

Effective magnetic correlations in hole-doped graphene nanoflakes

The magnetic properties of zig-zag graphene nanoflakes (ZGNF) are investigated within the framework of the dynamical mean-field theory. At half-filling and for realistic values of the local interaction, the ZGNF is in a fully compensated antiferromagnetic (AF) state, which is found to be robust against temperature fluctuations. Introducing charge carriers in the AF background drives the ZGNF metallic and stabilizes a magnetic state with a net uncompensated moment at low temperature. The change in magnetism is ascribed to the delocalization of the doped holes in the proximity of the edges, which mediate ferromagnetic correlations between the localized magnetic moments. Depending on the hole concentration, the magnetic transition may display a pronounced hysteresis over a wide range of temperature, indicating the coexistence of magnetic states with different symmetry. This suggests the possibility of achieving the electrostatic control of the magnetic state of ZGNFs to realize a switchable spintronic device.

cond-mat.str-el↗

arXiv2015

Tunable site- and orbital-selective Mott transition and quantum confinement effects in La$_{0.5}$Ca$_{0.5}$MnO$_3$ nanoclusters

We present a dynamical mean-field theory (DMFT) study of the charge and orbital correlations in finite-size La$_{0.5}$Ca$_{0.5}$MnO$_3$ (LCMO) nanoclusters. Upon nanostructuring LCMO to clusters of 3 nm diameter, the size reduction induces an insulator-to-metal transition in the high-temperature paramagnetic phase. This is ascribed to the reduction in charge disproportionation between Mn sites with different nominal valence [Das et al., Phys. Rev. Lett. 107, 197202 (2011)]. Here we show that upon further reducing the system size to a few-atom nanoclusters, quantum confinement effects come into play. These lead to the opposite effect: the nanocluster turns insulating again and the charge disproportionation between Mn sites, as well as the orbital polarization, are enhanced. Electron doping by means of external gate voltage on few-atom nanoclusters is found to trigger a site- and orbital-selective Mott transition. Our results suggest that LCMO nanoclusters could be employed for the realization of technological devices, exploiting the proximity to the Mott transition and its control by size and gate voltage.

cond-mat.str-el↗

arXiv2015

Dynamical vertex approximation in its parquet implementation: application to Hubbard nano-rings

We have implemented the dynamical vertex approximation (D$Γ$A) in its full parquet-based version to include spatial correlations on all length scales and in {\sl all} scattering channels. The algorithm is applied to study the electronic self-energies and the spectral properties of finite-size one-dimensional Hubbard models with periodic boundary conditions (nanoscopic Hubbard rings). From a methodological point of view, our calculations and their comparison to the results obtained within dynamical mean-field theory, plain parquet approximation, and the exact numerical solution, allow us to evaluate the performance of the D$Γ$A algorithm in the most challenging situation of low dimensions. From a physical perspective, our results unveil how non-local correlations affect the spectral properties of nanoscopic systems of various sizes in different regimes of interaction strength.

cond-mat.str-el↗

arXiv2013

Correlation effects in transport properties of interacting nanostructures

We discuss how to apply many-body methods to correlated nanoscopic systems, and provide general criteria of validity for a treatment at the dynamical mean field theory (DMFT) approximation level, in which local correlations are taken into account, while non-local ones are neglected. In this respect, we consider one of the most difficult cases for DMFT, namely for a quasi-one-dimensional molecule such as a benzene ring. The comparison against a numerically exact solution shows that non-local spatial correlations are relevant only in the limit of weak coupling between the molecule and the metallic leads and of low inter-atomic connectivity, otherwise DMFT provides a quantitative description of the system. As an application we investigate the role of correlations on electronic transport in quantum junctions, and we show that a local Mott-Hubbard crossover is a robust phenomenon in sharp nanoscopic contacts.

cond-mat.str-el↗

arXiv2012

Double Exchange model for nanoscopic clusters

We solve the double exchange model on nanoscopic clusters exactly, and specifically consider a six-site benzene-like nanocluster. This simple model is an ideal testbed for studying magnetism in nanoclusters and for validating approximations such as the dynamical mean field theory (DMFT). Non-local correlations arise between neighboring localized spins due to the Hund's rule coupling, favoring a short-range magnetic order of ferro- or antiferromagnetic type. For a geometry with more neighboring sites or a sufficiently strong hybridization between leads and the nanocluster, these non-local correlations are less relevant, and DMFT can be applied reliably.

cond-mat.str-el↗

arXiv2011

Size control of Charge-Orbital Order in Half-Doped Manganite, La$_{0.5}$Ca$_{0.5}$MnO$_3$

Motivated by recent experimental results, we study the effect of size reduction on half-doped manganite, La$_{0.5}$Ca$_{0.5}$MnO$_3$, using the combination of density functional theory (DFT) and dynamical mean field theory (DMFT). We find that upon size reduction, the charge-ordered antiferromagnetic phase, observed in bulk, to be destabilized, giving rise to the stability of a ferromagnetic metallic state. Our theoretical results, carried out on defect-free nanocluster in isolation, establish the structural changes that follow upon size reduction to be responsible for this. Our study further points out the effect of size reduction to be distinctively different from application of hydrostatic pressure. Interestingly, our DFT+DMFT study, additionally, reports the correlation-driven stability of charge-orbitally ordered state in bulk La$_{0.5}$Ca$_{0.5}$MnO$_3$, even in absence of long range magnetic order.

cond-mat.str-el↗

arXiv2010

Fourier transformation and response functions

We improve on Fourier transforms (FT) between imaginary time $τ$ and imaginary frequency $ω_n$ used in certain quantum cluster approaches using the Hirsch-Fye method. The asymptotic behavior of the electron Green's function can be improved by using a "sumrule" boundary condition for a spline. For response functions a two-dimensional FT of a singular function is required. We show how this can be done efficiently by splitting off a one-dimensional part containing the singularity and by performing a semi-analytical FT for the remaining more innocent two-dimensional part.

cond-mat.str-el↗